Internal combustion engine designers have increasingly come to realize that substantially improved fuel supply systems are required in order to obtain higher levels of pollution abatement and increased fuel economy. Among the known options, direct fuel injection improves performance but higher initial costs have tended to discourage its general adoption. Additionally, only the more sophisticated and more expensive direct injection systems are capable of achieving the increasingly higher performance goals of engine manufacturers.
Early fuel injection systems centered on distributor type fuel injection systems having a single centralized high pressure pump and a distributor valve for metering and timing fuel flow from the pump to each of a plurality of injection nozzles, such as disclosed in U.S. Pat. No. 3,557,765. Although simple in design concept, these systems generally suffer defects inherent with separation of the injector nozzles from the centralized pump. Unit injector systems avoid these inherent defects by providing each engine cylinder with its own cam-actuated pump such as disclosed in U.S. No. 3,544,008. However, except for heavy duty compression ignition engine applications, the performance advantage of unit injectors has generally not outweighed the detriment of greater costs. The design of a commercially competitive unit fuel injector therefore normally involves acceptance of some characteristics which are less than optimal since the basic goals of low cost, high performance, and reliability are often in direct conflict.
One method of improving the performance characteristics of unit fuel injectors at relatively low cost is to vary the injection rate. A recent method for varying the injection rate is disclosed in commonly owned U.S. Pat. No. 3,965,875 entitled "Fuel Injection System for Diesel Engines," which discloses a fuel injector in which fuel is injected at a relatively slow rate during a first phase of the injector plunger injection stroke and then is injected at a faster rate during a second phase of the injector plunger injection stroke. During the initial phase of injection, the injection rate is slower, due to the compression of an auxiliary spring mounted around a rod which is received within and reciprocates the injection plunger. The auxiliary spring reduces the overall spring rate of the drive train and slows the inward movement of the plunger for a predetermined compression, after which fuel is injected at a faster rate.
Fenne, U.S. Pat. No. 3,718,283, discloses a fuel injection system including a coaxial spring biased valve which allows a substantially unrestricted flow of fuel from a fuel pump through orifices for fuel injection following an initial period of restricted flow. The restricted flow and unrestricted flow are defined by the distance between the valve and its seat adjacent the orifices as controlled by fuel pressure acting on the combination of springs.
Another patent to Fenne, U.S. Pat. No. 3,747,857, discloses a fuel injection system for restricting the initial flow of fuel through an injector orifice where fuel pressure is used to urge a valve away from its initial seated position of closing off the injection orifices to a first position permitting restricted flow and a second position further from the seat allowing greater or unrestricted flow. The fuel pressure acting on the valve to cause the two positions is controlled by a relief valve and a spring biased piston within a chamber, wherein the piston moves against the spring during the initial restricted flow until the piston movement is stopped causing the fuel pressure acting on the valve to increase and to move the valve to a position of unrestricted flow.
Although the systems discussed above create different stages of injection, further improvement is desirable. None of these patents discloses a unit fuel injector that adequately simultaneously controls engine noise, nitric oxide emissions, and unburned hydrocarbons by controlling the rate of pressure rise.